Preheat oven to 425°F. Lightly grease a baking sheet, set aside.
In a large mixing bowl combine the flour with the baking powder, the salt, the baking soda and the sugar. Blend together the peanut butter with the egg white, then add the milk. Whisk well and stir in the flour mixture. Start to whisk with a fork to combine the butter mixture with the flour and then start to knead with your hands until reach a workable dough. Pour dough onto the prepared baking sheet and flat it lightly with your hands or just with fingers. Cut into 8 wedges Brush with milk and sprinkle with some extra sugar. Pour into the prepared oven and bake about 12 - 15 minutes. Cool on a wire rack.